Virtual Machine (VM)

• Give the application an entire isolated “machine”, down to the (virtual) bare silicon
• Run an entire operating system on this
• Run the untrusted application on that OS
• It can be very safe

How VMs work

• Recall the hardware access control mechanisms: privileged operations and memory protection
• Run the guest operating system unprivileged
• Any time the guest OS issues a privileged operation, it traps to the virtual machine monitor (VMM)
• The VMM emulates the operation. For example, an attempt at disk I/O is mapped to I/O to a real file that represents the virtual disk

VM Security

• Very strong isolation
• Very high overhead. . .
  • Must set up and administer an entire OS
  • Guest copies of Microsoft Windows require just as many patches as do native copies
• Performance can be bad, though some hardware architectures have special instructions to improve VM performance.

Interacting with a VM

• Often don’t want perfect isolation.
• Example: cut-and-paste between windows
• Performance can be dramatically enhanced if the guest OS signals the VMM
• Example: add a virtual “graphics” driver that calls the VMM, via the equivalent of a system call

Limitations of Virtual Machines

• Would you let your enemy put a machine inside your data center?
• VMs can spread viruses, launch DoS attacks, etc.
• VMs require just as much care, administration, and monitoring as do real machines
• In many situations, they represent an economic mechanism rather than a security mechanism (Save on power, cooling, etc.)
• But — may be less painful when wiping the disk and starting over